Electrostatic spray coating apparatus



Dec. 30, 1969 R, mm ET AL 3,486,483

ELECTROSTATIC SPRAY COATING APPARATUS Original Filed April 12, 1967 3Sheets-Sheet 1 R. TILNEY ET AL ELECTROSTATIC SPRAY'COATING APPARATUSDec. 30, 1969,

3 Sheets-Sheet 2 Original Filed April 12, 1967 E QE @ AZV/ Nfl////////%/ Q .Q A m U E r\\/\ \m N W S R. TILNEY ET AL ELECTROSTATICSPRAY COATING APPARATUS Original Filed April 12, 1967 Dec. 30,1969

3 Sheets-Sheet 5 N wE United States Patent Int. Cl. Bb 5/02 a v US. Cl.118-626 i i Claims ABSTRACT OF ,THEDISCLOSURE A safe electrostaticcoatingapparatuscapableof producing a spray pattern having an adjustableWidth achieved by a laminated resistive blade assembly. received in ahollow body, and forming with the body a shallow recess. The hollow bodyforms a coating material reservoir and has a series of coating materialopenings spaced from the blade. In operation the blade is chargedthrough a resistive connection and coating material is forced from thereservoir through the opening to flow over theouter surface of the bodyinto the recess and to'form a thin film on the surface of the blade fromwhich it is atomized by the electrostatic field to the article anddeposited upon the article. Adjustment of spray width is achieved byobstructing a portion of the openings in the body.

This application is a continuationof application Ser. No. 630,236, filedApr. 12, 1967, now abandoned.

This invention relates to electrostatic spray coating apparatus of thekind in which a liquid coating material 'to be atomized is fed to theedge of an elongated member, between which and an article to be coated,a highvoltage electrostatic field is maintained to cause atomization ofthe coating material and to promote its deposition upon the article byelectrostatic attraction.

Features and advantages of the invention'will appear from the followingdescription of an embodimentthereof; given by way of example, inconjunction with the a'c'com Panying drawings, in which: FIGURE 1A is asectional plan-viewof a'liquid'coating atomizing and charging apparatus;I

FIGURE 1B is a sectional view of a body member cooperatively associatedwiththe apparatus of FIGURE 1Ai and r FIGURE 2 taken on the line'22 ofFIGURE 1;

The apparatus which is shown in these drawings is suitable for coatingan article or succession'of articles presenting a surface to be coatedwhich is elongated and of roughly constant width. Thus, the apparatuscan be used for coating webs of material, or-for coating a shecession oftray-like'articles 'on a conveyor. In either case,

and coating apparatus.

The apparatus comprises a main body "member of hollow cylindrical formfrom which extends in an ap-' proximately radial direction a bladeassembly 11. Coating material is fed to the interior of the body' at 12and is exuded from the body through a series of parallel spaced radialopenings 13; the exuded material falls by gravity over the outersurfaceof the body and-onto the surface of the blade assembly, forming a thinfilm along the knifelike edge of the assembly. A high voltage is causedto appear at this edge with respect to the article or articles beingcoated and by this means the liquid coating material is atomized anddeposited. I v

The body member 10 is formed of insulating material and at one end,which is the inlet end for liquid coating the article or articles is orare moved past the atomizing ice material, is closed by an inlet plug14, having a shoulder 15 which abuts against the end of the body and isheld by a plug fixing cap 16, threaded onto the end of the body. Thebody is provided with drain plug openings closed by drain plugs 17 and18. In FIGURE 1, the drain plugs are shown angularly displaced, forpurposes of illustration.

, The other end of the body member 10 is closed by an end tube assembly.The end of the body is counterbored at 20 to receive the end of an endplug 21, which is stepped in diameter to fit within the counterbored endof the body, and is there secured by adhesive such as an epoxy resin.The surface of the end plug is cylindrical, and fitting on this is anend tube 22, the end of this tube being reduced at 23 to fit Within afurther counterbored portion 24 of the main body. The end tube 22 isheld on the end plug 21 by means of a locknut 25, engaging the threadedend of plug 21 with an interposed washer 26. Together the end tube 22and the main body member 10 present an approximately cylindricalexternal surface. With the end of the body member 10 closed by plug 21in the way described, and with the inserted inlet plug 14,

' there is formed a reservoir within the body member for coatingmaterial, and the material can be fed to this reservoir through a feedinlet passage 27, to which connection can be made at 28 to an externalsupply of coating material. This coating material is fed by means of ametering pump so as accurately to maintain the desired rate of feed ofcoating material to the blade assembly.

To elfect electrostatic atomization, high voltage which may be in therange of to 100 kv., must be impressed upon the atomizing device in sucha way as to establish high voltage at the edge of the blade assembly. Itis desirable that the arrangement should be such that the danger ofshock or of fire hazard due to excessive or disruptive discharges fromthe edge of the blade assembly should be avoided. With this object, theblade assembly is made in a way which presents a low effective capacity,in the sense described in US. Patent 3,048,498, and the voltage isestablished at the edge of the blade through a circuit which hasefiectively a high resistance.

The means for this purpose includes a voltage entry assembly, indicatedgenerally at 30 in FIGURE 1, ex-

tending from the main body assembly substantially at right angles.Voltage from a suitable high voltage source is fed through a highvoltage cable 31 which passes into the assembly 30, the centralconductor 32 being attached to the end of a high value resistor 33. Thecable and resistor are mounted in a termination unit which includes ahousing tube 34 into one end of which the cable 31 is inserted andsecured; the tube has a restricted portion 35 through which conductor 32extends, the conductor being attached to a spring 36in turn contactingthe end of rethreaded to engage the end tube housing 44.

The high voltage connection from the final contact spring 39 to the edgeof the blade is made in the following way. The spring 39 enters a recess47 formed in the end tube 22; the recess is relatively narrow andextends along the length of the end tube 22 as far as a peripheralrecess at 46. In the base of recess 45 and around the peripheral recess46 is laid a coating of material which has a high electrical resistance,such as a carbon or graphite loaded synthetic resin material. Once thismaterial has been placed in position and allowed to harden, the

major part of the elongated recess 45 can be closed and sealed by means48, which can be an insert or merely a filling of suitable resin.

To receive the blade assembly 11, the main body member is formed with anelongated longitudinal slot 50, of a width to receive the blade assemblyin the manner shown in FIGURE 2. The end of this recess breaks into theinner surface of the counterbored end portion of the main body member,so that at the end of the slot 50 the peripheral recess 46 in the endtube 22 is exposed. The blade assembly 11 is formed of two flat stripmembers 51 and 52 which have sandwiched between them a layer 53 ofresistive material, which can be, for example, a fibre glass clothimpregnated with carbon or graphite. The two strip members 51 and 52 aresecurely fixed together by an epoxy resin adhesive at 54 and 55 and alsoat intermediate localized areas such as 56, where the resin may extendthrough holes formed in the fibre glass material. The strips 51 and 52are thereby securely fastened together and the exposed edges of thestrips are shaped to a knife edge as shown more clearly in FIG- URE 2,on the surface of which the edge of the intermediate layer 53 will beexposed. The blade assembly is fastened in recess 50 by means of acarbon loaded resin at 57 so that there will be a high resistanceelectrical circuit on the exposed edge of strip 53, through resin 57,the resin in peripheral recess 46 and thus through the recess 45 back tothe contact spring 39, and thence through resistor 33 to the conductorof cable 31.

In operation, liquid coating material is supplied by the metering pumpto the reservoir 12 and the liquid will emerge slowly through theopenings 13. Size, spacing and rate of flow of liquid are so adjustedthat the liquid, moving by gravity over the outer cylindrical surface ofthe body assembly, will establish at the forward knife edge of theassembly a substantially uniform, thin film of the coating liquid and onthis thin film there will appear a high voltage from the supplyconnected to cable 31. The liquid is finely atomized and is drawn off bythe electrostatic field in the customary manner of electrostaticdeposition. By reason of the inclusion of the relatively high resistancematerial in the layer 53 and the high resistance connections extendingback to resistor 33, the system will be safe inasmuch as it will besubstantially free of danger of fire or shock hazard.

The apparatus described can be varied in a number of ways. For example,depending upon the nature and especially the resistivity, of the coatingmaterial, it may be arranged that the current path over which highvoltage is established at the atomizing edge is provided largely orwholly by the body of the material. One way of doing this is to mount awire within, and preferably concentrically within, the reservoirprovided by space 12. Also, to supplement or replace this arrangement,or that described with reference to the drawings, the interior walls ofthe space 12 can be coated with a resistive coating material.

In some circumstances it may be preferable to construct the blade sothat a single thickness of insulating material is coated on one sideonly with resistive material, but the sandwich construction is usuallymore advantageous; it can, for example, easily be sharpened, to give theknife edge required for atomization, if the edge is damaged charging canalso be effected by a fine gauge copper wire 59 embedded in the bladenear the junction of the blade and the body.

To vary the rate of flow and distribution of coating material, otherthan the line of holes described can be used. Slots, disposed along thelength of the cylindrical body can be used and the slots can be parallelto, or slightly inclined to the length of the body. With the same objectof ensuring good flow control of the coating material, a long recess 60can be cut in the external cylindrioal surface of the body, and coatingmaterial into the recess fed from the interior reservoir through holesleading into the bottom of the recess. The recess will serve tostabilize the flow. This stabilizing effect of a trough or groove can beotherwise used; a shallow groove or recess 58 can be provided, extendinggenerally across the flow path of the material, between the deliveryholes or recess and the atomizing edge, a convenient position for such agroove is at the junction of the blade and the body.

For uniformity of coating thickness, it is desirable that the materialshould be atomized from a length of blade which is about the same as thewidth of the article .or articles being coated. For a semi-permanentinstallation the blade length will about equal the width of thearticles, but to adapt a blade for use with narrow articles the feed canbe reduced, and limited to part only of the total length of the blade.One method of limiting the feed in this way is by the use of blockswhich fit or slide over the exterior of the body and obstruct the exitopenings for the coating material. Conveniently, the blocks surround thebody but for the blade; if coating material is fed from a groove orrecess, the blocks may lodge in the recess. The block can be of the sameor a similar insulating material as that used for the body; polyethyleneis suitable.

The blade arrangement can be different. The central body can supportanother blade, the two blades emerging from the body on opposite sidesand with their upper surfaces at an angle that gives a suitable gravityflow. The central closed reservoir can be replaced by an open troughwith an over-spill feed to the blade or blades. The two blade systemobviously gives an increased rate of coating.

It has been assumed that the article being coated is approximately flat,but if articles of other cross-sectional shapes are being coated betterdeposition may be obtained by changing the shape of the atomizing edge.For example, in coating material which is longitudinally, considered inthe direction of movement of the blade with respect to the article,channelled or corrugated the blade may be given an interrupted edgeportion of which correspond approximately to the corrugations on thelike discrete separated lengths of blade can be used with thisarrangement. It is important that in use sufiicient field intensity ismaintained at the edge, to give adequate atomizing and deposition.

Depending to a large extent on the nature of the coating material, theremay be advantage in oscillating or vibrating the .blade assembly. Lowspeed oscillation may give improved lateral uniformity of coating insome application and faster speeds up to vibrations speeds of the orderof alternating current supply mains frequency will assert distributionand flow of the coating material over the blade. Vibration may extendthe use of the blade to powdered material. The blade can be heated, toimprove the flow characteristics of the material, especially liquidmaterial, or to assist polymerization of resin-type coating materials.

We claim:

1. An electrostatic spray coating apparatus for atomizing and depositingliquid coating material upon an article to be coated comprising a hollowbody member of insulating material sealed at both ends to form areservoir for coating material with a coating material inlet, a bladeassembly comprising a strip of insulating material and resistivematerial received in the body and forming at its junction with the bodya shallow recess, a series of radial openings in the body spaced fromthe blade assembly, a high voltage entry housing of insulating materialextending from the body member and enclosing a high voltage conductor,means carried by the body connecting one end of the high voltageconductor to the resistive material of the blade assembly, and liquidsupply means connected with the coating material inlet to exude coatingmaterial from the openings to flow over the surface of the body into therecess to form a thin film on the blade assembly.

2. The apparatus of claim 1 wherein the body is sealed at the end withan end tube assembly including a plug and an end tube having a recess,and means carried by the body connecting one end of the highvoltage'conductor to the resistive material of the blade assemblyincludes resistive material carried by the recess in the end tube.

3. The apparatus of claim 1 wherein the body is cylindrical, the bladeassembly includes two strips of insulating material with the resistivematerial between the two strips, and a fine wire is embedded in theblade near the junction of the blade and the body.

4. The apparatus of claim 1 wherein a recess extends over the exteriorsurface of the body and over the radial openings.

5. An electrostatic spray coating apparatus for atomizing and depositingliquid coating material upon an article to be coated comprising a hollowbody member of insulating material sealed at both ends to form areservoir for coating material with a coating material inlet, a bladeassembly comprising a strip of insulating material and resistivematerial received in the body, means to permit coating material to exudefrom said reservoir, means to form the coating material into a thinfilm, a high voltage entry housing of insulating material extending fromone end of the body and enclosing a resistance, electrical connectionmeans carried by said body connecting said resistance with saidresistive material of said blade ris sembly, plug means of insulatingmaterial sealing said one end of the reservoir and including a portionof said eletrical connection means and liquid supply means con- .nectedwith the coating material inlet to exude coating UNITED STATES PATENTS2,651,287 9/ 1953 Turner. 2,695,002 11/ 1954 Miller. 2,748,018 5/ 1956Miller. 3,048,498 8/1962 Juvinall et al. 3,246,844 4/ 1966 Lehman et al.3,380,845 4/1968 Shapiro et al.

PETER FELDMAN, Primary Examiner US. or. X.R. 239 -15

